1. Field of the Invention
The present invention is directed to self-threading fittings for electrical conduits and more specifically to a self-threading fitting having a tubular guide sleeve of insulating material.
2. Prior Art
In the past, when it was desired to provide a pipe thread on the end of an unthreaded length of conduit or pipe a fitting was utilized which was provided with external threads at one end and no threads at the other end. Such a fitting was secured to the end of the conduit by means of a slip-fit joint and retained thereon by means of set screws. Such an arrangement has several drawbacks, namely, that it is difficult to provide a fluid-tight seal between the fitting and the conduit and depending upon the location of the fitting and conduit it is often difficult to obtain suitable access to the set screws which must be driven in a direction transverse to the axis of the conduit.
Another well known type of fitting is the so-called self-threading fitting. The use of self-threading fittings, per se, similar to the fitting disclosed in the present application are old and well known in the art. However, a principle problem with such fittings has been the ability to thread the self-threading fitting on the end of a conduit in perfect coaxial alignment. Although the fitting and the conduit may be initially aligned with some degree of accuracy by visual observation it is almost impossible to maintain such alignment during the application of torque to the fitting as it is threaded onto the end of the conduit.
The use of an insulating lining within the fitting for an electrical conduit is also old and well known. However, in the past such liners were always assembled permanently to the fitting by means of adhesives, heatsealing or the like and would therefore not be generally suitable for field installation.
In the threaded coupling art it is old and well known to provide an elongated tubular extension on one part of the threaded coupling which will act as a pilot or guide sleeve during the threading engagement of a threaded fitting with another threaded fitting. Such extensions are generally integral with the coupling part and are therefore relatively expensive to manufacture. While such a pilot or guide sleeve may assist in bringing the two fittings toward each other for engagement they are not utilized to insure axial alignment during threading engagement since both fittings are prethreaded.